Method of manufacturing MEMS devices providing air gap control

ABSTRACT

Methods and apparatus are provided for controlling a depth of a cavity between two layers of a light modulating device. A method of making a light modulating device includes providing a substrate, forming a sacrificial layer over at least a portion of the substrate, forming a reflective layer over at least a portion of the sacrificial layer, and forming one or more flexure controllers over the substrate, the flexure controllers configured so as to operably support the reflective layer and to form cavities, upon removal of the sacrificial layer, of a depth measurably different than the thickness of the sacrificial layer, wherein the depth is measured perpendicular to the substrate.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 11/478,702, filed Jun. 30, 2006, entitled METHOD OF MANUFACTURINGMEMS DEVICES PROVIDING AIR GAP CONTROL which is hereby incorporated byreference in its entirety herein and is further related to co-pendingU.S. Application entitled METHOD OF MANUFACTURING MEMS DEVICES PROVIDINGAIR GAP CONTROL 12/436,059.

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates tomicroelectromechanical systems for use as interferometric modulators.More particularly, this invention relates to improved methods ofmanufacturing microelectromechanical system devices having differentsized cavities between a movable element and a substrate.

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF THE INVENTION

An embodiment provides for a method of making at least two types ofmicroelecromechanical systems (MEMS) devices, the at least two types ofMEMS devices having different release states after removal ofsacrificial material, the method including providing a substrate,forming a first electrically conductive layer over at least a portion ofthe substrate, forming a first sacrificial layer over at least a portionof the first conductive layer, forming a plurality of electricallyconductive moveable elements over the first sacrificial layer andforming a plurality of flexure controllers over the substrate configuredso as to operably support the electrically conductive moveable elementswhen the sacrificial layer is removed, and where the first sacrificiallayer is removable to thereby release the MEMS devices and form cavitieshaving at least two gap sizes between the first electrically conductivelayer and the movable elements.

Another embodiment provides a method of making at least two types ofinterferometric modulators, the at least two types of interferometricmodulators having different cavity depths after removal of a sacrificialmaterial, the method including providing a substrate, forming an opticalstack over at least a portion of the substrate, forming a firstsacrificial material over at least a portion of the optical stack,wherein the sacrificial material is removable to thereby form cavities,forming a second electrically conductive layer over portions of thefirst sacrificial material, and forming at least two types of flexurecontrollers over the substrate, the flexure controllers configured so asto operably support the second electrically conductive layer, whereinthe at least two types of flexure controllers comprise different sizedcomponents, the different sized components configured to form cavitiesof different depths under the portions of the second electricallyconductive layer after removal of the first sacrificial layer.

Another embodiment provides a microelecromechanical system (MEMS) deviceincluding a substrate, a plurality of moveable elements over thesubstrate, each moveable element separated from the substrate by acavity, and a plurality of flexure controllers over the substrateconfigured so as to operably support the moveable elements, wherein theplurality of flexure controllers comprise portions of differentdimensions to control selected flexures. The selected flexures serve toform the cavities having at least two gap sizes between the substrateand the plurality of movable elements.

Another embodiment provides a method of controlling a depth of a cavitybetween two layers of a device that includes one or more thin filmlayers, the method including providing a substrate, forming asacrificial layer over at least a portion of the substrate, forming afirst layer over at least a portion of the sacrificial layer, andforming one or more flexure controllers over the substrate, the flexurecontrollers configured so as to operably support the first layer and toform cavities, upon removal of the sacrificial layer, of a depth about30% greater or more than the depth of the sacrificial layer, wherein thedepth is measured perpendicular to the substrate.

Another embodiment provides an unreleased microelecromechanical system(MEMS) device that includes a substrate, a sacrificial layer over atleast a portion of the substrate, a moveable element over the firstsacrificial layer, and one or more flexure controllers over thesubstrate configured so as to operably support the moveable element andto form a cavity between the substrate and the movable element, uponremoval of the sacrificial layer, of a depth about 30 percent greater ormore than the depth of the sacrificial layer, wherein the depth ismeasured perpendicular to the substrate, the sacrificial layer beingremovable by etching.

Another embodiment provides a method of controlling a depth of a cavitybetween two layers of a device comprising one or more thin film layers,the method including providing a substrate, forming a sacrificial layerover at least a portion of the substrate, the sacrificial layer beingremovable by etching, forming a first thin film layer over at least aportion of the sacrificial layer, and forming one or more flexurecontrollers over the substrate, the flexure controllers configured so asto operably support the first thin film layer and to displace the thinfilm layer towards the substrate, upon removal of the sacrificial layer.

Another embodiment provides an unreleased microelecromechanical system(MEMS) device, that includes a substrate, a sacrificial layer over atleast a portion of the substrate, a moveable element over the firstsacrificial layer and one or more flexure controllers over the substrateconfigured so as to operably support the moveable element and todisplace the movable element towards the substrate, upon removal of thesacrificial layer, the sacrificial layer being removable by etching.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a flow diagram illustrating certain steps in an embodiment ofa method of making an interferometric modulator.

FIGS. 9A-9G are schematic cross sections illustrating certain steps in aprocess for fabricating an interferometric modulator having post supportstructures.

FIGS. 10A-10D are schematic cross sections illustrating certain steps ina process for fabricating an interferometric modulator having rivetsupport structures.

FIG. 11 is a flow diagram illustrating certain steps in an embodimentfor fabricating an interferometric modulator having flexure controllers.

FIG. 12A-12K show cross sections of alternative embodiments ofinterferometric modulators having different flexure controllers that canbe fabricated using the method of FIG. 11.

FIGS. 13A-13F show results of analytical studies designed to show theeffects that altering the characteristics of flexure controllerstructures can have on a deflection of a supported layer upon release ofthe device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

An embodiment provides methods of making MEMS devices with a pluralityof flexure controllers over a substrate. The flexure controllers areconfigured so as to operably support electrically conductive moveableelements and to provide a plurality of selected flexures when asacrificial layer is removed. The sacrificial layer is removable tothereby release the MEMS devices and form cavities having at least twogap sizes. The flexure controllers can effectuate increases in gap sizeas well as decreases in gap sizes. As a result, multiple depositions,masking and etching steps may be replaced by fewer deposition, maskingand etching steps, thus saving time and money in the manufacture of MEMSdevices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise of several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack are patterned intoparallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the relaxed or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be relaxed areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or relaxed pre-existingstate. Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding, and vacuum forming. In addition, the housing 41 may be madefrom any of a variety of materials, including but not limited toplastic, metal, glass, rubber, and ceramic, or a combination thereof. Inone embodiment the housing 41 includes removable portions (not shown)that may be interchanged with other removable portions of differentcolor, or containing different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g. filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28, and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a memory device such as a digital video disc (DVD) or a hard-discdrive that contains image data, or a software module that generatesimage data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. Those of skill in the art will recognizethat the above-described optimization may be implemented in any numberof hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support postsare formed of a planarization material, which is used to form supportpost plugs 42. The embodiment illustrated in FIG. 7E is based on theembodiment shown in FIG. 7D, but may also be adapted to work with any ofthe embodiments illustrated in FIGS. 7A-7C as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

FIG. 8 is a flow diagram illustrating certain steps in an embodiment ofa manufacturing process 800 for an interferometric modulator. Such stepsmay be present in a process for manufacturing, e.g., interferometricmodulators of the general type illustrated in FIGS. 1 and 7, along withother steps not shown in FIG. 8. With reference to FIGS. 1, 7 and 8, theprocess 800 begins at step 805 with the formation of the optical stack16 over the substrate 20. The substrate 20 may be a transparentsubstrate such as glass or plastic and may have been subjected to priorpreparation step(s), e.g., cleaning, to facilitate efficient formationof the optical stack 16. As discussed above, the optical stack 16 iselectrically conductive, partially transparent and partially reflective,and may be fabricated, for example, by depositing one or more of thelayers onto the transparent substrate 20. In some embodiments, thelayers are patterned into parallel strips, and may form row electrodesin a display device. In some embodiments, the optical stack 16 includesan insulating or dielectric layer that is deposited over one or moremetal layers (e.g., reflective and/or conductive layers).

The process 800 illustrated in FIG. 8 continues at step 810 with theformation of a sacrificial layer over the optical stack 16. Thesacrificial layer is later removed (e.g., at step 825) to form thecavity 19 as discussed below and thus the sacrificial layer is not shownin the resulting interferometric modulator 12 illustrated in FIGS. 1 and7. The formation of the sacrificial layer over the optical stack 16 mayinclude deposition of a XeF2-etchable material such as molybdenum oramorphous silicon, in a thickness selected to provide, after subsequentremoval, a cavity 19 having the desired size. Deposition of thesacrificial material may be carried out using deposition techniques suchas physical vapor deposition (PVD, e.g., sputtering), plasma-enhancedchemical vapor deposition (PECVD), thermal chemical vapor deposition(thermal CVD), or spin-coating.

The process 800 illustrated in FIG. 8 continues at step 815 with theformation of a support structure e.g., a post 18 as illustrated in FIGS.1 and 7 or a rivet as discussed below. The formation of the post 18 mayinclude the steps of patterning the sacrificial layer to form a supportstructure aperture, then depositing a material (e.g., a polymer orsilicon dioxide) into the aperture to form the post 18, using adeposition method such as PECVD, thermal CVD, or spin-coating. In someembodiments, the support structure aperture formed in the sacrificiallayer extends through both the sacrificial layer and the optical stack16 to the underlying substrate 20, so that the lower end of the post 18contacts the substrate 20 as illustrated in FIG. 7A. In otherembodiments, the aperture formed in the sacrificial layer extendsthrough the sacrificial layer, but not through the optical stack 16. Forexample, FIG. 7C illustrates the lower end of the support post plugs 42in contact with the optical stack 16. More detailed discussions of otherembodiments providing for formation of posts and rivets are presentedbelow.

The process 800 illustrated in FIG. 8 continues at step 820 with theformation of a movable reflective layer such as the movable reflectivelayer 14 illustrated in FIGS. 1 and 7. The movable reflective layer 14may be formed by employing one or more deposition steps, e.g.,reflective layer (e.g., aluminum, aluminum alloy) deposition, along withone or more patterning, masking, and/or etching steps. As discussedabove, the movable reflective layer 14 is typically electricallyconductive, and may be referred to herein as an electrically conductivelayer. Since the sacrificial layer is still present in the partiallyfabricated interferometric modulator formed at step 820 of the process800, the movable reflective layer 14 is typically not movable at thisstage. A partially fabricated interferometric modulator that contains asacrificial layer may be referred to herein as an “unreleased”interferometric modulator.

The process 800 illustrated in FIG. 8 continues at step 825 with theformation of a cavity, e.g., a cavity 19 as illustrated in FIGS. 1 and7. The cavity 19 may be formed by exposing the sacrificial material(deposited at step 810) to an etchant. For example, an etchablesacrificial material such as molybdenum or amorphous silicon may beremoved by dry chemical etching, e.g., by exposing the sacrificial layerto a gaseous or vaporous etchant, such as vapors derived from solidxenon difluoride (XeF2) for a period of time that is effective to removethe desired amount of material, typically selectively relative to thestructures surrounding the cavity 19. Other etching methods, e.g. wetetching and/or plasma etching, may also be used. Since the sacrificiallayer is removed during step 825 of the process 800, the movablereflective layer 14 is typically movable after this stage. After removalof the sacrificial material, the resulting fully or partially fabricatedinterferometric modulator may be referred to herein as a “released”interferometric modulator.

In one embodiment, support structures may take the form of poststructures (e.g., posts 18 shown in FIGS. 1 and 7) underlying themovable layer. An exemplary process for fabricating an interferometricmodulator comprising support posts is discussed with respect to FIGS.9A-9G. In various embodiments, fabricating an interferometric modulatorcomprises forming an optical stack on a substrate, which may be alight-transmissive substrate, and in further embodiments is atransparent substrate. The optical stack may comprise a conductivelayer, which forms an electrode layer on or adjacent the substrate; apartially reflective layer, which reflects some incident light whilepermitting some light to reach the other components of theinterferometric modulator element; and a dielectric layer, whichinsulates the underlying electrode layer from the other components ofthe interferometric modulator. In FIG. 9A, it can be seen that atransparent substrate 100 is provided, and that a conductive layer 102and a partially reflective layer 104 are deposited over the substrate100. A dielectric layer 106 is then deposited over the partiallyreflective layer 104. In some embodiments, the conductive layer 102 istransparent and comprises ITO, the partially reflective layer 104comprises a semireflective thickness of metal, such as chromium (Cr),and the dielectric layer 106 comprises silicon oxide (SiO₂). Thedielectric layer may also be a stack comprising SiO₂ and Al₂O₃. At somepoint during this process, at least the conductive layer 102 ispatterned (not shown) to form row electrodes which will be used toaddress a row of interferometric modulators. In one embodiment, thispatterning takes place after the deposition of the conductive andpartially reflective layers 102 and 104, but prior to the deposition ofthe dielectric layer 106. The combination of the layers 102, 104, and106 is referred to as the optical stack 110, and may be indicated by asingle layer in later figures, for convenience. It will be understoodthat the composition of the optical stack 110 may vary both in thenumber of layers and the components of those layers, and that the layersdiscussed above are merely exemplary.

A variety of methods can be used to perform the patterning and etchingprocesses discussed with respect to the various embodiments disclosedherein. The etches used may be either a dry etch or a wet etch, and maybe isotropic or anisotropic. Suitable dry etches include, but are notlimited to: SF₆/O₂, CHF₃/O₂, SF₂/O₂, CF₄/O₂, and NF₃/O₂. Generally,these etches are suitable for etching one or more of SiO_(x), SiN_(x),SiO_(x)N_(y), spin-on glass, Nissan Hard coat, and TaO_(x), but othermaterials may also be etched by this process. Materials which areresistant to one or more of these etches, and may thus be used as etchbarrier layers, include but are not limited to Al, Cr, Ni, and Al₂O₃. Inaddition, wet etches including but not limited to PAD etches, BHF, KOH,and phosphoric acid may be utilized in the processes described herein.Generally, these etches may be isotropic, but can be made anisotropicthrough the use of a reactive ion etch (RIE), by ionizing the etchchemicals and shooting the ions at the substrate. The patterning maycomprise the deposition of a photoresist (PR) layer (either positive ornegative photoresist), which is then used to form a mask. Alternately, ahard mask can be utilized. In some embodiments, the hard mask maycomprise metal or SiNx, but it will be understood that the compositionof the hard mask may depend on the underlying materials to be etched andthe selectivity of the etch to be used. In The hard mask is typicallypatterned using a PR layer, which is then removed, and the hard mask isused as a mask to etch an underlying layer. The use of a hard mask maybe particularly advantageous when a wet etch is being used, or wheneverprocessing through a mask under conditions that a PR mask cannot handle(such as at high temperatures, or when using an oxygen-based etch).Alternate methods of removing layers may also be utilized, such as anashing etch or lift-off processes.

In FIG. 9B, it can be seen that a layer 112 of sacrificial material isdeposited over the optical stack 110. In one embodiment, thissacrificial layer 112 comprises molybdenum (Mo), but in otherembodiments, the sacrificial layer 112 may comprise other materials,such as, for example, amorphous silicon (a-Si). In FIG. 9C, thesacrificial layer 112 has been patterned and etched to form taperedapertures 114, which correspond to the locations of post or supportregions. These apertures 114 are advantageously tapered in order tofacilitate continuous and conformal deposition of overlying layers.

In FIG. 9D, a layer 118 of post material is deposited over the patternedsacrificial layer 114, such that the post layer 118 also coats the sidewalls and the base of the tapered apertures 114. In certain embodiments,the post layer 118 may comprise silicon nitride (SiN_(x)) or SiO₂,although a wide variety of other materials may be used. In FIG. 9E, thepost layer 118 is patterned and etched to form posts 120. It can be seenin FIG. 9E that the edges of the posts 120 preferably taper which, likethe tapered or sloped sidewalls of the apertures 114, facilitatecontinuous and conformal deposition of overlying layers.

In FIG. 9F, a highly reflective layer 122 is deposited over the posts120 and the exposed portions of the sacrificial layer 112. A mechanicallayer 124 is then deposited over the highly reflective layer 122. Forconvenience, the highly reflective layer 122 and the mechanical layer124 may be referred to and depicted in subsequent figures as adeformable reflective layer 130 (see FIG. 9G), whenever the mechanicallayer 124 is deposited directly over the highly reflective layer 122. Inalternate embodiments, the deformable reflective layer 130 may comprisea single layer which has the desired optical and mechanical properties.For example, mechanical or moving layers for mechanical switches neednot include reflective layers. Since the sacrificial layer 112 is stillpresent at this stage of the process 200, the mechanical layer ordeformable reflective layer 130 is typically not yet movable. Apartially fabricated MEMS device 135, e.g. a partially fabricatedinterferometric modulator, that contains a sacrificial layer (the layer112 in this embodiment) may be referred to herein as an “unreleased”MEMS device.

In FIG. 9G, a release etch is performed to remove the sacrificial layer112, forming an interferometric modulator element 140 having aninterferometric cavity 19 through which the deformable reflective layer130 can be moved in order to change the color reflected by the releasedinterferometric modulator element 140. Prior to the release etch, thedeformable reflective layer 130 is preferably patterned to form columns(not shown), and may advantageously be further patterned to form etchholes (not shown) which facilitate access to the sacrificial layer bythe release etch.

In another embodiment support structures may take the form of rivetstructures overlying the mechanical or deformable reflective layer 130.A process for forming overlying rivet structures is discussed anddepicted with respect to FIGS. 10A-10D.

In one embodiment, this process includes the steps of FIGS. 9A-9C. InFIG. 10A, it can be seen that a mechanical layer or deformablereflective layer 130 is deposited over the patterned sacrificial layer112, such that the deformable reflective layer 130 coats the side wallsand base of the tapered apertures 114.

In FIG. 10B, a rivet layer 142 is deposited over the deformablereflective layer 130. The rivet layer 142 may comprise, for example,SiO₂, SiN_(x), or Ni, but a wide variety of alternate materials may beutilized for the rivet layer 142. Next, in FIG. 10C, the rivet layer ispatterned and etched to form rivet structures 150. Since the sacrificiallayer 112 is still present at this stage of the process 200, themechanical layer or deformable reflective layer 130 is typically not yetmovable. A partially fabricated MEMS device 135, e.g. a partiallyfabricated interferometric modulator, that contains a sacrificial layer(the layer 112 in this embodiment) may be referred to herein as an“unreleased” MEMS device. In FIG. 10D, it can be seen that thesacrificial layer 112 has been removed via a release etch, permittingthe deformable reflective layer 130 to be able to move through theinterferometric cavity 19 of the released interferometric modulator 140.

It will be understood that additional support may be provided through acombination of posts 120 (FIG. 9G) and rivets 150 (FIG. 10D). Forinstance, rivets 150 may be formed in some locations within aninterferometric modulator, and posts 120 may be formed at others, orrivets 150 may be formed overlying the posts 120.

In the process described with respect to FIGS. 9A-9G, it can be seenthat the sacrificial layer 112 is exposed to the etching process whichpatterns the inorganic post 120 (see FIG. 9E) and the support post 120is similarly exposed to the release etch which removes the sacrificiallayer 112 (see FIG. 9G). Unless modifications are made to the processflow, the support post material 118 should be selectively etchablerelative to the sacrificial material, and vice versa. In addition evenif an etch exists which will selectively etch one relative to another,alternate etches which are not selective may be preferable for otherreasons.

Flexure of the support structures and the mechanical layer may occur asa result of unbalanced stresses within the support structures and themechanical layer. In some situations, these unbalanced stresses are theresult of inherent stresses within the materials forming the supportstructures and the mechanical layer, which are a function of thematerials comprising those layers. An additional source of unbalancedstresses is the thermal expansion of the layers, which is a function ofthe mismatch between the coefficients of thermal expansion of twodifferent materials, the operating temperature of the MEMS device, themoduli of elasticity of the materials, and the material depositionconditions. When adjoining layers have different coefficients of thermalexpansion, deflection may not only be caused by the relative change insize of adjoining layers, but the total deflection may vary as theresult of the operating temperature. Because such deflection will alterthe height of the interferometric cavity, and therefore affect the colorreflected by the interferometric modulator element, it is desirable totake this flexure into account in manufacturing interferometricmodulator elements with different cavity heights. In one embodiment, asingle thickness sacrificial layer is applied, rather than multipledepositions of sacrificial material corresponding to the multiple cavityheights, and posts and/or rivets exhibiting differing flexures willproduce multiple cavity heights upon release of the interferometricmodulators.

FIG. 11 is a flow diagram illustrating certain steps in an embodiment ofa method of making a device such as a MEMS device having a cavity. Suchsteps may be present in a process for manufacturing, e.g.,interferometric modulators of the general type illustrated in FIGS. 1and 7, along with other steps not shown in FIG. 11. Many of the steps ofthe process in FIG. 11 are similar to steps depicted schematically inFIGS. 9 and 10. The process of FIG. 11 may be used to manufacture MEMSdevices such as the various unreleased and released interferometricmodulators depicted in FIGS. 12A through 12K. The devices shown in FIG.12 include flexure controllers that will produce multipleinterferometric cavity heights while requiring fewer deposition, maskingand etching steps. With reference to FIGS. 9, 10, 11 and 12, the process200 begins at step 205 where a substrate 100 is provided. In oneembodiment, the substrate 100 may comprise any transparent material suchas glass or plastic.

The process 200 continues at step 210 with the formation of a firstelectrically conductive layer 102 on the substrate 100 as shown in FIG.9A. The first electrically conductive layer 102 can be a single layerstructure or multiple sub-layer structure as described above.

The process 200 continues at step 215 with the formation of one or moreother layers, e.g., a partially reflective layer 104, and a dielectriclayer 106 over at least a portion of the electrically conductive layer105 as shown in FIG. 9A. The combination of the layers 102, 104, and 106is referred to as the optical stack 110 as shown in FIG. 9B and FIG. 12.

The process 200 continues at step 220 with the formation of asacrificial layer 112 as shown in FIG. 9B. In FIGS. 9, 10 and 12Athrough 12H, a single sacrificial layer is formed between the deformablereflective layer 130 (e.g., including both the highly reflective layer122 and the mechanical layer 124) and the optical stack 110. In theinterferometric modulators shown in FIG. 12I, 12J and 12K, a firstsacrificial layer 112A is formed over the optical stack 110 prior to theformation of a movable reflective layer 14 (including the highlyreflective layer 122) at step 225. In the embodiments shown in FIGS.12I, 12J and 12K, the movable reflective layer 14 may be considered as amovable element that is suspended over the substrate by a mechanicallayer 34. Without loss of generality, the term movable element willherein be used to describe any movable element in a MEMS device, e.g., amovable or deformable reflective layer 130 as shown in FIGS. 9 and 10,any of the movable reflective layers 14, 14 a or 14 b as shown in FIGS.1 and 7, or the movable elements 14 as shown in FIGS. 12I, 12J and 12K.The movable element 14 may be formed by a deposition followed bypatterning and etching. After forming the movable element 14, a secondsacrificial layer 112B is deposited over the movable element 14.Subsequent patterning and etching of the second sacrificial layer 112B(or the single sacrificial layer 112) may performed to form supportstructure apertures 114 as shown in FIGS. 9C and 10A as well asapertures for attaching the mechanical layer 34 to the movable element14 as shown in FIG. 12. In a preferred embodiment only one deposition isperformed in forming the sacrificial layer 112 (or 112A) between themovable reflective element 14 (as shown in FIG. 12) or the deformablereflective layer 130 (as shown in FIGS. 9 and 10) and the substrate.

In an embodiment of an interferometric modulator, the sacrificial layeris deposited so as to form, upon subsequent removal, an interferometriccavity with a thickness in the range of about 1000 angstroms to about5000 angstroms between the movable layer 14 or the deformable reflectivelayer 130 and the optical stack 16 of FIGS. 1, 7 and 12.

In the dual sacrificial layer embodiments shown in FIGS. 12I, 12J and12K, the process 200 continues at step 230 with the formation of amechanical layer 34 over at least a portion of the sacrificial layer112B and at least a portion of the movable element 14. In the singlesacrificial layer embodiment of FIGS. 9 and 10, the mechanical layer 34is replaced by the mechanical layer 124 that is formed over the highlyreflective layer 122. The mechanical layers 34 and 124 may be comprisedof the same or different materials.

The process continues at step 235 where flexure controllers are formed.In the exemplary process 200 shown in FIG. 11, a plurality of flexurecontrollers having different dimensions are formed in order to providemultiple cavity sizes after removal of the sacrificial layer. In anotherembodiment, a flexure controller is formed to provide a desired cavitysize that is either smaller or larger than the cavity prior to removalof the sacrificial layer. The flexure controllers, e.g., post structuresand/or rivet structures, induce displacement of a membrane to which theflexure controllers are attached (e.g., the deformable reflective layer130), subsequent to removal of the sacrificial layer(s) 112. Details ofsome exemplary flexure controllers will be discussed below.

The process 200 continues at step 240 where the sacrificial layer 112 isremoved (e.g., by etching) to form a cavity 19 as shown in FIG. 10G. Theremoval of the sacrificial layers can be accomplished, for example, byexposure to an etchant such as XeF₂, F₂ or HF alone or in combination.In a preferred embodiment, substantially all of the sacrificial layer112 is removed in the etching process. In one embodiment, the cavity 19is an interferometric cavity between an optical stack 110 and thedeformable reflective layer 130. After formation of the cavity 19, theresulting MEMS device, e.g., the interferometric modulator, is in a“released” state.

Some examples of flexure controllers that may be formed at step 235 ofthe process 200 will now be discussed. For example, FIG. 12A shows anunreleased device, e.g., an interferometric modulator, which includesposts 120 having a wing portion of dimension 122 substantially parallelto the substrate 100 and the deformable reflective layer 130. Thesacrificial layer 112 has a thickness 126 as measured perpendicular tothe substrate 100 and optical stack 110. FIG. 12B shows the device afterremoval of the sacrificial layer 112 forming the cavity 19. The releaseddevice of FIG. 12B has a cavity depth of 128A as measured perpendicularto the substrate 100 and optical stack 110. The depth of the cavitybetween the released deformable layer 130 and the optical stack 110(shown as Ref. No. 128A) is measurably larger in this example than theunreleased cavity depth of 126 shown in FIG. 12A. The difference incavity depth is due to the flexure controlled by the combined stressesof the posts 120 and the deformable reflective layer 130.

FIG. 12C shows a second example of an unreleased device, e.g., aninterferometric modulator, which includes posts 120 having a wingportion of dimension 124 substantially parallel to the substrate 100 andthe deformable reflective layer 130. In this example, the sacrificiallayer 112 has approximately the same thickness 126 as measuredperpendicular to the substrate 100 and optical stack 110 as the deviceshown in FIG. 12A. However, the overlap 124 of FIG. 12C is larger thanthe overlap 122 of FIG. 12A. The overlaps 122 and 124 of the posts 120are the result of patterning and etching steps as discussed above andshown in FIG. 9E. FIG. 12D shows the device of FIG. 12C after removal ofthe sacrificial layer 112 forming the cavity 19. The released device ofFIG. 12D has a cavity depth of 128B as measured perpendicular to thesubstrate 100 and optical stack 110. The depth of the cavity between thereleased deformable layer 130 and the optical stack 110 (shown as Ref.No. 128B) is measurably larger in this example than the unreleasedcavity depth of 126 shown in FIGS. 12A and 12C and larger than thereleased cavity depth 128A shown in FIG. 12B. The difference in cavitydepth is due to the flexure controlled by the combined stresses of theposts 120 (having the overlap 124 compared to the overlap 122 of FIG.12A) and the deformable reflective layer 130.

FIGS. 12E and 12G show examples of a devices wherein the flexurecontrollers comprise rivets 150 (as discussed above and shown in FIG.10) overlying the deformable reflective layer 130. The rivets 150 ofFIG. 12E have a smaller overlapping portion (or wing) than the rivets150 of FIG. 12G (see dimensions 123 and 125). In this example, the depth127 of the sacrificial layer 112 is approximately the same for bothdevices. However, after release of the devices, the corresponding cavitydepths may vary significantly as depicted by the depth 129A of FIG. 12Fand the depth 129B of FIG. 12H.

FIGS. 12I, 12J and 12K depict examples of unreleased interferometricmodulators with various flexure controlling post structures 120 andrivet structures 150. FIG. 12I has rivet structures 150 overlying themechanical layer 34 and post structures underlying the mechanical layer34 where the rivets 150 and posts 120 have similar overlap. The rivetstructures 150 of FIG. 12J, exhibit much less overlap while the poststructures 120 exhibit more overlap. FIG. 12K depicts a device where therivet structures have significantly more overlap that the poststructures 120.

During fabrication of interferometric modulators, upward flexures ofmovable reflective layers, upon releasing of the device (as depicted inFIGS. 12B and 12D), of about 500 angstroms or less have been observed.However, downward flexures of movable reflective layers, upon releasingof the device (as depicted in FIGS. 12F and 12I), typically never occur.By varying the size and/or material of which the flexure controllers,e.g., posts and/or rivets, are comprised, increased upward flexureand/or downward flexure of membranes may be achieved. For example,depositing thinner post and/or rivet layers may result in less upwardflexure or increased downward flexure. Forming flexure controllers ofmore rigid materials may result in less flexure. Decreasing tensilestress in an overlying flexure controller, e.g., a rivet, may reduceupward flexure. Decreasing tensile stress in an underlying flexurecontroller, e.g., a post, may increase upward flexure. Tensile stressestend to shrink the portion of the device in which they are contained. Incontrast, compressive stresses tend to expand the portion of the devicein which they are contained. One of skill in the art will recognize thatby varying the relative sizes of posts 120 and/or rivets 150 as well asvarying materials of which the posts 120 and/or rivets 150 arecomprised, significantly different released cavity depths may beachieved. Ranges of flexure, upward or downward, including about 50 to100, about 100 to 150, about 150 to 200, about 200 to 250, about 250 to300, about 300 to 350, about 350 to 400, about 400 to 450, about 450 to500, about 500 to 550, about 550 to 600, about 600 to 650, about 650 to700, about 700 to 750, about 750 to 800, about 800 to 850, about 850 to900, about 900 to 950, about 950 to 1000, about 1000 to 1050, about 1050to 1100, about 1100 to 1150, about 1150 to 1200 angstroms or more may beachieved by varying sizes and/or material properties of flexurecontrollers as discussed above. In addition, increments or decrements tothese ranges of about 5, 10, 15, 20, and 25 angstroms may be possible.

The methods described herein for controlling cavity depth of MEMSdevices may have a positive effect on the manufacture of various devicesincluding MEMS devices that comprise cavities, e.g., interferometricmodulators. For example, Table 1 summarizes the results of a set ofexperiments in which various post structure overlaps were fabricated ininterferometric modulators having similar unreleased sacrificial layerdepths. Post structure overlaps, similar to the overlaps 122 and 124depicted in FIGS. 12A and 12C respectively, were varied from 1 micron to3 microns for interferometric modulator pixels measuring 222 microns by222 microns. The thickness of the sacrificial layer in these experimentswas about 1150 angstroms. After releasing the interferometricmodulators, the undriven cavity depth (as measured perpendicular to thesubstrate) between the movable element and the optical stack variedsignificantly.

TABLE 1 Undriven Cavity Depth Post Overlap (μm) (Angstroms) 1.0 1400 2.01775 2.5 2000 3.0 2200

Relatively small changes in post overlap resulted in more than 50%variation in undriven cavity depth from the shallowest to the deepestcases shown in Table 1. By varying dimensions and or materials of postsand/or rivets as discussed above, even larger variations may bedemonstrated. The post structures used in the test resulted in increasesin gap size due to the tensile stress in the post structures (See FIGS.12B and 12D). However, by utilizing rivet structures and/or combinationsof post and rivet structures, decreases or sagging of the deformablereflective layer (as depicted in FIGS. 12F and 12H) may also berealized. As discussed above, cavity depths of about 1000 angstroms toabout 5000 angstroms are desirable for interferometric modulators. Arange of cavity sizes from about 2000 angstroms to about 4000 angstromsis preferred for modulating visible light while smaller and/or largercavity sizes may be used for modulating hyperspectral light, ultravioletlight and/or infrared light. Increases in cavity depth of about 30% to40%, about 40% to 50%, about 50% to 60%, about 60% to 70%, about 70% to80%, about 80% to 90%, about 90% to 100% or more may be achieved. Inaddition, increments or decrements to these ranges of about 1%, 2%, 3%,4% and 5% may be obtained.

In addition to the experiments discussed above that have shown theeffect that various structures of flexure controllers have on cavitydepth, analytical studies have also been made that simulate theexperiments and indicate that additional capability of controlling thecavity depth may also be afforded. FIGS. 13A-13F show results ofanalytical studies designed to show the effects that altering thecharacteristics of flexure controller structures can have on adeflection of a supported layer upon release of the device. Theanalytical equations used in the studies model the effects of stresses,both tensile and compressive, contained in the various rivets and/orpost structures when combined with the stresses contained in a layerthat they are supporting. The modeled stresses contained in the supportstructures and in the supported layer represent stresses that may resultdepending on the conditions under which the different layers are formed.Compressive stresses, designated by negative stress levels in thestudies, tend to expand the portion of the device in which they arecontained. Tensile stresses, designated by positive stress levels in thestudies, tend to shrink the portion of the device in which they arecontained. The studies looked at various combinations of posts and/orrivet structures. The studies also modeled the effect that ranges ofcertain dimensions and/or characteristics of different portions of theflexure controller structures have on the resulting deflection. Thedimensions and characteristics of the flexure controller structures thatwere analyzed include layer thickness, overlap length, and stress levelof the various portions of the device. The analysis modeled the flexurecontroller posts and/or rivets and the supported layer as cantileveredbeams. The structures used in the analysis are representative of any ofseveral types of devices including, but not limited to, MEMS devices,light modulating devices, and any device comprising one or more thinfilm layers having a cavity between one of the thin film layers and thesubstrate and/or between two of the thin film layers.

The results shown in FIGS. 13A-13F will be discussed in relation to theinterferometric modulator embodiments shown in FIG. 12. It should benoted that the interferometric modulator is an example of a device thatmay be modeled using the analytical methods presented here, and otherdevices may also be analyzed and manufactured using the various methodsdescribed above.

The configuration of the device analyzed in the first example includes a1000 angstrom thick (as measured perpendicular to the substrate 100)deformable reflective layer 130 comprised of Ni. The Ni layer is modeledwith a 400 MPa tensile stress that is representative of the type ofstress levels seen under typical deposition conditions. The device alsoincludes a 2000 angstrom thick (as measured perpendicular to thesubstrate) oxide post structure 120. The oxide post structures modeledin the analysis comprised SiO₂. The post structure overlaps thedeformable reflective layer 130 by 3 μm, where the overlap is measuredas depicted by the dimensions 122 and 124 shown in FIGS. 12A and 12C.The post structure is modeled with a −400 MPa compressive stress. Thedevice also includes an oxide rivet structure 150, where the thicknessof the rivet structure (as measured perpendicular to the substrate) is1000 angstroms for FIG. 13A and is variable on the horizontal axis forFIG. 13B. The rivet structure overlaps the deformable reflective layer130 by 3 μm, where the overlap is measured as depicted by the dimensions123 and 125 shown in FIGS. 12E and 12G. The stress of the rivetstructure is variable on the horizontal axis for the analysis resultsshown in FIG. 13B and is −400 MPa for the analysis results shown in FIG.13B. FIGS. 13A and 13B show the resulting deflections, upon release ofthe sacrificial layer 112 resulting in the cavity 19, of the deformablereflective layer 130. Positive deflection values represent deflectionsaway from the substrate 100 as depicted in FIGS. 12B and 12D. Negativedeflection values represent deflection towards the substrate 100 asdepicted in FIGS. 12F and 12H.

The results of FIG. 13A show that for increased compressive stress (morenegative values), the deflection is lower, showing an estimateddeflection of just over 300 angstroms for a −500 MPa oxide rivet stress.As the compressive stress is reduced to zero, the deflections becomelarger, showing an estimated deflection greater than 800 angstroms for azero MPa stress level. Deflections of even greater values can beobtained by forming rivet structures with tensile stress levels(positive stress). The reason that all the deflections are positive(away from the substrate) in the example of FIG. 13A is that thecombined oxide post stress of −400 MPa and the deformable reflectivelayer stress of 400 MPa both contribute to a positive deflection thatthe oxide rivet stress levels analyzed do not overcome. Smaller valuesof deflection, including negative deflection values, could be obtainedby several methods, including applying more negative compressive rivetstress levels, reducing the thickness of the compressed oxide post,reducing the compressive stress of the oxide post, reducing thethickness of the compressed oxide post, increasing the thickness of thecompressed oxide rivet, reducing the overlap length of the compressedoxide post, increasing the overlap length of the compressed rivet andother methods known to those of skill in the art. These methods allserve to reduce the energy levels of portions of the device contributingto upward deflections (e.g., the compressed post 120 and the stretcheddeformable reflective layer 130) and/or increase the energy levels ofportions of the device contributing to downward deflections (e.g., thecompressed rivet 150).

The results of FIG. 13B show that as the thickness of the compressedoxide rivet 150 is increased (increasing the energy contributing to adownward deflection), the deflection, upon release of the sacrificiallayer 112, is reduce, even becoming a negative value for rivetthicknesses greater than about 2000 angstroms.

The next example includes a 2000 angstrom thick oxide post 120 with acompressive stress of −400 MPa, a 1000 angstrom thick oxide rivet 150with a compressive stress of −400 MPa, and a 1000 angstrom thickdeformable reflective Ni layer 130 with a tensile stress of 400 MPa. Theoverlap lengths of the post 120 (see the dimensions 122 and 124 in FIGS.12A and 12C) and the rivet 150 (see the dimensions 123 and 125 in FIGS.12E and 12G) are equal and are varied from about 2 μm to about 6 μm.FIG. 13C shows the estimated deflections of the deformable reflectivelayer 130 upon release of the sacrificial layer 112. Increasing theoverlap lengths of both the post 120 and the rivet 150 increasesdeflections away from the substrate 100. As in the cases above, thecompressed post 120 and the stretched deformable reflective layer 130both contribute to the upward deflections and the compressed rivet 150contributes to a downward deflection. In this case, the combinedenergies of the layer 130 and the post 120 outweigh the energy of therivet 150 and the deflections are all positive. By varying the overlaplengths of the oxide post 120 and the oxide rivet 150 from about 2 μm toabout 6 μm, the deflection of the layer 130 can be varied from about 200angstroms to abut 1700 angstroms.

FIG. 13D shows deflections of the deformable reflective layer 130, uponrelease of the sacrificial layer 112, for a case similar to that of FIG.13C except that there is no oxide rivet. In this example, the positivedeflections of the deformable reflective layer 130 are even greater thanwithout the rivet because the compressed rivet is not working againstthe upward deflections caused by the post 120 and the deformablereflective layer 130. By varying the overlap length of the oxide post120 from about 2 μm to about 6 μm, the deflection of the layer 130 canbe varied from about 500 angstroms to abut 5500 angstroms.

FIG. 13E shows deflections of the deformable reflective layer 130, uponrelease of the sacrificial layer 112, for a case similar to that of FIG.13D (including no oxide rivet) but with a fixed oxide post 120 overlapof 3 μm (see dimensions 122 and 124 in FIGS. 12A and 12C) and variedoxide post stress levels. In this example, the positive deflections ofthe deformable reflective layer 130 decrease for less negative values(lower compressive stress levels) of oxide post stress. For acompressive oxide post stress level of 500 MPa, the upward deflection ofthe deformable reflective layer 130, upon release of the sacrificiallayer 112, is about 1600 angstroms, and is about 350 angstroms for azero stress level.

The final example includes a 2000 angstrom thick oxide post 120 with acompressive stress of −400 MPa, a 1000 angstrom thick oxide rivet 150with a compressive stress of −200 MPa, and a 1000 angstrom thickdeformable reflective Ni layer 130 with a tensile stress of 400 MPa. Theoverlap lengths of the post 120 (see the dimensions 122 and 124 in FIGS.12A and 12C) and the rivet 150 (see the dimensions 123 and 125 in FIGS.12E and 12G) are equal and are varied from about 2 μm to about 6 μm.FIG. 13F shows the estimated deflections of the deformable reflectivelayer 130 upon release of the sacrificial layer 112. Increasing theoverlap lengths of both the post 120 and the rivet 150 increasesdeflections away from the substrate 100. As in the cases above, thecompressed post 120 and the stretched deformable reflective layer 130both contribute to the upward deflections and the compressed rivet 150contributes to a downward deflection. In this case, the combined stresslevels of the layer 130 and the post 120 outweigh the stress level ofthe rivet 150 and the deflections are all positive. By varying theoverlap lengths of the oxide post 120 and the oxide rivet 150 from about2 μm to about 6 μm, the estimated deflection of the layer 130 variesfrom about 250 angstroms to abut 2500 angstroms.

The analytical studies of the examples discussed above show thatvariation in dimensions and/or characteristics of the various portionsmaking up the flexure controller structures and/or other layers canaffect the deflection of a supported layer upon release of the device.Those of skill in the art will be able to recognize other ways ofmodifying portions of similar types of devices in order to alter thecavity depth of the released device.

An embodiment of an unreleased interferometric modulator includes firstmeans for reflecting light, second means for reflecting light, firstmeans for supporting the second reflecting means, wherein the firstsupporting means is removable by etching, and second means forsupporting the second reflecting means and for forming a cavity betweenthe first reflecting means and the second reflecting means, upon removalof the first supporting means, of a depth about 30 percent greater ormore than the depth of the first supporting means, wherein depth ismeasured perpendicular to the first reflecting means. With reference toFIGS. 9 and 12, aspects of this embodiment include where the firstreflecting means is a partially reflective layer 104, where the secondreflecting means is a movable reflective layer 14, where the firstsupporting means is a sacrificial layer 112, and where the secondsupporting means is at least one of a post structure 120 and a rivetstructure 150.

Another embodiment of an unreleased interferometric modulator includesfirst means for reflecting light, second means for reflecting light,first means for supporting the second reflecting means, and second meansfor supporting the second reflecting means and for effecting adisplacement of the second reflecting means towards the first reflectingmeans upon removal of the first supporting means, wherein the firstsupporting means is removable by etching. With reference to FIGS. 9 and12, aspects of this embodiment include where the first reflecting meansis a partially reflective layer 104, where the second reflecting meansis a movable reflective layer 14, where the first supporting means is asacrificial layer 112, and where the second supporting means is at leastone of a post structure 120 and a rivet structure 150.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. An electromechanical device, comprising: a substrate; a plurality ofmoveable elements over the substrate, each moveable element separatedfrom the substrate by a cavity; and a plurality of flexure controllersover the substrate configured so as to operably support the moveableelements, wherein the flexure controllers comprise wing portions, eachwing portion being connected to a portion of a movable element which itsupports, and wherein the wing portions of at least one of the pluralityof flexure controllers underlies the portion of the moveable elementsthat it is connected to and supports.
 2. The device of claim 1, furthercomprising an electrically conductive layer over at least a portion ofthe substrate.
 3. The device of claim 1, wherein at least a portion ofthe plurality of moveable elements are electrically conductive.
 4. Thedevice of claim 1, wherein at least a portion of the plurality ofmoveable elements comprise a reflective surface.
 5. The device of claim1, wherein the plurality of flexure controllers are formed from one ormore of silicon nitride (SiN) and silicon dioxide (SiO₂).
 6. Anelectromechanical device, comprising: a substrate; a plurality ofmoveable elements over the substrate, each moveable element separatedfrom the substrate by a cavity; and a plurality of flexure controllersover the substrate configured so as to operably support the moveableelements, wherein the flexure controllers comprise wing portions, eachwing portion being connected to a portion of a movable element which itsupports, and wherein the plurality of flexure controllers comprise atleast two flexure controllers having wing portions of differing length.7. The device of claim 6, wherein the plurality of flexure controllersare formed from one or more of silicon nitride (SiN) and silicon dioxide(SiO₂).
 8. The device of claim 6, wherein at least a portion of theplurality of moveable elements comprise a reflective surface.
 9. Anelectromechanical device, comprising: a substrate; a plurality ofmoveable elements over the substrate, each moveable element separatedfrom the substrate by a cavity; and a plurality of flexure controllersover the substrate configured so as to operably support the moveableelements, wherein the flexure controllers comprise wing portions, eachwing portion being connected to a portion of a movable element which itsupports, and wherein at least a portion of at least two of theplurality of flexure controllers comprise wing portions of differingthickness.
 10. The device of claim 9, wherein the plurality of flexurecontrollers are formed from one or more of silicon nitride (SiN) andsilicon dioxide (SiO₂).
 11. The device of claim 9, wherein at least aportion of the plurality of moveable elements comprise a reflectivesurface.
 12. An electromechanical device, comprising: a substrate; aplurality of moveable elements over the substrate, each moveable elementseparated from the substrate by a cavity; and a plurality of flexurecontrollers over the substrate configured so as to operably support themoveable elements, wherein the flexure controllers comprise wingportions, each wing portion being connected to a portion of a movableelement which it supports, and wherein at least two of the plurality offlexure controllers comprise different materials.
 13. The device ofclaim 12, wherein the plurality of flexure controllers are formed fromone or more of silicon nitride (SiN) and silicon dioxide (SiO₂).
 14. Thedevice of claim 12, wherein at least a portion of the plurality ofmoveable elements comprise a reflective surface.
 15. Anelectromechanical device, comprising: a substrate; a plurality ofmoveable elements over the substrate, each moveable element separatedfrom the substrate by a cavity; and a plurality of flexure controllersover the substrate configured so as to operably support the moveableelements, wherein the flexure controllers comprise wing portions, eachwing portion being connected to a portion of a movable element which itsupports, and wherein the flexure controllers further comprise taperededges connected to the wing portions, and a base which operably supportsthe edges and separates the edges by a selected distance.
 16. The deviceof claim 15, wherein the plurality of flexure controllers are formedfrom one or more of silicon nitride (SiN) and silicon dioxide (SiO₂).17. The device of claim 15, wherein at least a portion of the pluralityof moveable elements comprise a reflective surface.
 18. Anelectromechanical device, comprising: a substrate; a film layer over atleast a portion of the substrate layer; one or more flexure controllersover the substrate operably supporting the film layer and separating thefilm layer from the substrate, the flexure controllers comprising wingportions that contact the film layer; one or more cavities definedbetween the substrate and the film layer; and wherein the plurality offlexure controllers induce displacement of the film layer such that whenthe flexure controller is in a relaxed position, at least one cavity hasa first depth which is about 30 percent or more greater than the depthof the cavity when the flexure controllers are positioned approximatelyparallel to the substrate in a non-released position, and wherein atleast one of the plurality of flexure controllers is formed underlyingthe film layer.
 19. The device of claim 18, wherein at least two of theplurality of flexure controllers comprise different materials.
 20. Anelectromechanical device, comprising: a substrate; a film layer over atleast a portion of the substrate layer; one or more flexure controllersover the substrate operably supporting the film layer and separating thefilm layer from the substrate, the flexure controllers comprising wingportions that contact the film layer; one or more cavities definedbetween the substrate and the film layer; and wherein the plurality offlexure controllers induce displacement of the film layer such that whenthe flexure controller is in a relaxed position, at least one cavity hasa first depth which is about 30 percent or more greater than the depthof the cavity when the flexure controllers are positioned approximatelyparallel to the substrate in a non-released position, and wherein thewing portions of at least two of the plurality of flexure controllershave wing portions of different lengths.
 21. An electromechanicaldevice, comprising: a substrate; a film layer over at least a portion ofthe substrate layer; one or more flexure controllers over the substrateoperably supporting the film layer and separating the film layer fromthe substrate, the flexure controllers comprising wing portions thatcontact the film layer; one or more cavities defined between thesubstrate and the film layer; and wherein the plurality of flexurecontrollers induce displacement of the film layer such that when theflexure controller is in a relaxed position, at least one cavity has afirst depth which is about 30 percent or more greater than the depth ofthe cavity when the flexure controllers are positioned approximatelyparallel to the substrate in a non-released position, and wherein atleast a portion of at least two of the plurality of flexure controllershave different thickness.
 22. An electromechanical device, comprising: asubstrate; a film layer over at least a portion of the substrate layer;one or more flexure controllers over the substrate operably supportingthe film layer and separating the film layer from the substrate, theflexure controllers comprising wing portions that contact the filmlayer; one or more cavities defined between the substrate and the filmlayer; and wherein the plurality of flexure controllers inducedisplacement of the film layer such that when the flexure controller isin a relaxed position, at least one cavity has a first depth which isabout 30 percent or more greater than the depth of the cavity when theflexure controllers are positioned approximately parallel to thesubstrate in a non-released position, and wherein the plurality offlexure controllers are formed from one or more of silicon nitride(SiN), and silicon dioxide (SiO₂).
 23. An electromechanical device,comprising: a substrate; a film layer over at least a portion of thesubstrate layer; one or more flexure controllers over the substrateoperably supporting the film layer and separating the film layer fromthe substrate, the flexure controllers comprising wing portions thatcontact the film layer; one or more cavities defined between thesubstrate and the film layer; and wherein the plurality of flexurecontrollers induce displacement of the film layer such that when theflexure controller is in a relaxed position, at least one cavity has afirst depth which is about 30 percent or more greater than the depth ofthe cavity when the flexure controllers are positioned approximatelyparallel to the substrate in a non-released position, and wherein theflexure controllers further comprise tapered edges connected to the wingportions, and a base which operably supports the edges and separates theedges of the flexure controllers by a selected distance.